The superior mechanical and strength-to-weight properties of carbon fibers has led to an important class of high performance fiber/matrix composites. These high performance composites are particularly useful for the production of aircraft and automobile body parts for which both strength and light weight are critical. Such composites enable manufacturers to produce relatively light weight structures without sacrificing strength. Consequently, much research has been directed to producing carbon fiber materials with ever increasing high performance properties and physical features that make them more valuable in commercial products and processes.
Various processes have been developed over the years for the production of high performance carbon fiber materials. One of the leading processes for producing high performance carbon fibers is the so-called PAN process wherein polyacrylonitrile (PAN) is used as a precursor fiber. The PAN process typically starts with a highly prestretched PAN fiber and consists of three steps. The first step is a stabilization treatment wherein the PAN fiber is heat treated in air at a temperature from about 200.degree. to 300.degree. C. for one or more hours. In the second step, the fiber is carbonized at a temperature above about 1100.degree. C. in a non-oxidizing atmosphere. The third step consists of a post heat treatment at temperatures up to about 2500.degree. C. to graphitize the fiber and give it its high performance properties. It is in this post heat treatment step that the chemical composition, the crystalline structure, and the mechanical properties, are strongly influenced.
Another method for producing high performance carbon fibers is referred to as the "ex-mesophase" method wherein pitch is spun into thread, then oxidized, carbonized, and graphitized. See for example French Patent No. 2,512,076.
High performance carbon fibers can also be prepared by a vapor-deposition method in which the fibers are produced by the thermal decomposition of a hydrocarbon on a substrate on which catalyst particles have been deposited. The catalyst particles are typically discrete particles of such metals as iron, cobalt, or nickel. One such vapor-deposition method teaches that carbon filaments are grown by exposing catalytic particles to a carburizing atmosphere. This is followed by exposing the filaments to an atmosphere whose carburizing potential is sufficiently high to deposit carbon from the vapor phase and thicken the filaments into a more longer and thicker fiber. See G. G. Tibbetts "Vapor-Grown Carbon Fibers", Carbon Fibers Filaments and Composites, edited by J. L. Figueiredo et. al., NATO ASI Series E: Applied Sciences, pages 73-94, Vol 177, 1989.
While high performance carbon fibers have met with a degree of commercial success as reinforcing materials in fiber/matrix composites, they nevertheless suffer from, inter alia, delamination problems. Several attempts have been made to modify carbon fibers to improve their interlaminar properties. For example, U.S. Pat. No. 4,816,289 teaches a method of producing crimped fibers. While crimped fibers have improved interlaminar properties, they would nevertheless still suffer from an unacceptable degree of delamination.
Another process teaches the formation of graphite fibers onto which secondary silicon carbide whiskers can be grown. While such a structure would show a substantial improvement in interlaminar shear strength, they unfortunately suffer from a number of shortcomings. For example, the silicon carbide whiskers are formed as only single non-branched structures. That is, there is no branching of the whiskers themselves, nor are the whiskers in a spiral or helical form. The carbon filaments which are produced on the carbon fibers in the practice of the present invention are branched, spiral, helical, or a combination thereof. These branched, spiral, and helical filament forms enhance interlocking of the fibers in the matrix. Furthermore, in the production of silicon carbide whiskers, relatively high temperatures (i.e., &gt;1000.degree. C.) are required. Still further, silicon carbide is intrinsically abrasive and thus leads to handling and processing problems. Other shortcomings include: (a) the thermal expansion coefficient of silicon carbide differs from that of carbon, and as a consequence, can initiate or propagate cracks in the resulting composite; (b) the densities of silicon carbide whiskers (ca. 3.22 g/cc) are considerably higher than those of carbon (2.25 g/cc); (c) at present, it is not possible to control silicon carbide whisker orientation and growth characteristics; and (d) the costs associated with producing secondary silicon carbide whiskers on carbon fibers is considerable. It is also believed that the bonding between the silicon carbide whiskers and the parent carbon fibers is non-chemical and thus would not be as strong as desired in certain applications.
Another process variation is taught in Sekiyu Gakkaishi, 28(5), 409-412, Egashira et. al, 1985, wherein carbon whiskers are grown on carbon fibers from the vapor phase catalyzed by iron sulfide. For example, the parent fibers are preoxidized with HNO.sub.3 at a temperature of about 120.degree. C. for one hour to facilitate supporting Fe on them. They are then impregnated with a 0.5 mol/L Fe(NO.sub.3).sub.3 solution, followed by reduction with hydrogen. A mixture of benzene, H.sub.2 S, and H.sub.2 are employed as the reactant gas. The whiskers, or filaments, produced are straight, non-branched filaments. That is, they cannot be characterized as being branched, spiral, or helical, as are the filaments of the present invention.
While the such methods of modifying carbon fibers do improve the interlaminar shear strength of the parent fibers to various degrees, there still exists a need in the art for high performance carbon fiber materials and structure with ever improved interlaminar properties and more economical ways of producing them.